在美国没有近亲，也没有打算让 朋友做 trust 的 successor.
那在 revocable trust 里，
1， 打算让老大 为 successor, 老二满18 岁后 为 co successor.
2， 限定老大在未到25 岁或未到30 不可以卖房子，股票，只可以支取收益部分。（ 比如债券利息，房租）
Successor trustee will become trustee if both parents passed away. So what you are asking is: can a trustee be the same as beneficiary?
The answer is yes. Trustee can be the same as beneficiary.
However, the downside would be since your oldest son controls the trust for himself for a few years, in case during those few years of sole trusteeship, he's sued for debt (causing injury for others, credit card debt etc), the creditor can reach into the trust because he's both trustee and beneficiary (for at least 1/2 of the trust).
You can tell the trustee how to invest, when to withdraw. It's your money it's up to you to put in whatever you want.
However, I am not sure an 18-25 year old is mature enough to follow the trust condition. For example, a few years ago if someone got dragged into bitcoin, it might be tempting to move some money out of trust to invest in bitcoin because they believe firmly they will make 10 times in return. I can see a 23 year old smartass kid do that in violation of trust requirement.
This is why the trust is usulaly managed by a trust lawyer or company for young adult beneficiaries.
Trust has to file tax every year, think of what's involved in a Trust before deciding their fee
Major banks have trust divisions, start there. Sorry I have no experience in this matter.
千万别认为美国有钱人的钱，会套成纸币像中国老农民和china town 的新移民一样放床底的鞋盒，又或是像小农思想的百姓一样存银行拿利息，呵呵。These kind of people won't become rich in the first place.